Paper is being put to increasingly diversed and specialized applications, and many attempts are made to improve paper quality by use of various kinds of quality-improving agents. However, use of such agents reduces, the friction coefficient of paper and has the disadvantage of making the paper more slippery.
Recently, paper-making has shifted away from the conventional acid paper-making method to alkaline paper-making method in which alkylketone dimer compounds are used as internal sizing agents, and it has been found that these compounds make the resulting paper slippery.
Slippery paper is troublesome to handle, since it often telescopically slips when rolled, and slides laterally when it is stacked.
Therefore, application of inorganic materials such as alumina, silica and the like on the surface of paper has been widely employed in order to roughen the surface for the prevention of slipping. However, this method causes pronounced abrasion of calendar rolls and requires constant inspection and maintenance of the equipment. Also, this method inevitably degrades the paper quality, especially the sized effect.
As surface treatment agents, an anti-slipping agent, a surface sizing agent, etc. are used as desired singly or in combination. However, when these agents are incompatible application, i.e., coating, must be carried out at different stations, which impairs efficiency of paper making.
Therefore, there is a demand for a surface-treating agent which provides paper with both antislipping property and sized property. Previously, we proposed a method for providing paper with anti-slipping property in which a cationic copolymer obtained by quaternization of a copolymer of a styrene compound and an aminoalkyl ester of acrylic acid or methacrylic acid (Japanese Laying-Open Patent Publication No.57-56598 (1982)). As surface sizing agents, property improvers, the following compounds are also disclosed in the following publications: a quaternized copolymer of styrene and nitrogen-containing vinyl monomer in U.S. Pat. No.2,964,445; a reaction product of epihalohydrin aqueous coating composition and a copolymer of a styrene compound and an aminoalkyl ester of (meth)acrylic acid in Japanese Laying-Open Patent Publication No.48-11407 (1973); and a quaternized copolymer of N,N-dimethylaminoethyl(meth)acrylate, styrene and acrylonitrile in Japanese Laying-Open Patent Publication No.56-118994(1981). However, these cationic copolymers have a defect in that they largely lose much of their sizing effect when the pH of the coating liquid is increased to 7-8 by any chemical reagent used in preparation of the coating liquid. Anionic surface-sizing agents commercially available today are not satisfactory in sizing and slip-prevention effects, either.
The purpose of the present invention is to provide a surface treatment agent for paper which has both sizing and slip-prevention effects, and are satisfactorily effective even when the pH of the coating liquid is 7 or higher.
We conducted an intensive study in search of such surface treatment agents and found that a quaternized product of a styrene/N,N-dialkylaminoalkyl(meth)acrylamide copolymer overcomes the above mentioned problems.